Omni and Astral Runes are unobtainable in Nightmare Dungeon

Before the introduction of Enhancers in the Nightmare Dungeon, you were able to obtain Omni and Astral Runes through a random 1% and 0.5% chance per mob death respectively. However, after the introduction of the Rune Enhancers (celestial, omni, and astral) it appears the runes are no longer obtainable in Nightmare Dungeon (or are obtainable but so rarely that it's basically impossible).